Histology
Loose connective tissue

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Loose Connective Tissue

Loose connective tissue, also known as areolar tissue, is a cellular connective tissue characterised by thin and relatively sparse collagen fibres. This type of tissue has a semi-fluid matrix with lesser proportions of fibres, making it more pliable and mesh-like compared to other connective tissues. It plays a very important role in the diffusion of oxygen and nutrients from capillaries, as well as in the removal of carbon dioxide and metabolic wastes back to the vessels.

This tissue is primarily located beneath the epithelia that cover body surfaces and line internal surfaces, and is also associated with glandular epithelium and surrounds the smallest blood vessels.

Histology of Loose Connective Tissue

Composition

Loose connective tissue consists of a fluid matrix that functions to cushion and protect body organs. The ground substance, primarily made up of collagenous and elastic fibres, occupies more volume than the fibres. This tissue contains widely dispersed fibroblasts, which are irregular branching cells that secrete strong fibrous proteins and proteoglycans as an extracellular matrix. The cells are generally connected by a gelatinous substance known as ground substance.

Cells

The cells that make up loose connective tissue include:

• Fibroblasts: These cells secrete collagen proteins to help maintain the structural framework of tissues.
• Macrophages: White blood cells that surround and kill microorganisms, remove dead cells, and stimulate the action of other immune system cells.
• Mast cells: Play a role in inflammatory responses by releasing histamine and heparin.
• Adipocytes: Fat cells that store energy in the form of lipids.
• Leukocytes: Various types of white blood cells involved in the immune response.
• Plasma cells: Produce antibodies as part of the immune reaction.

Fibres

Loose connective tissue is named based on the "weave" and type of its constituent fibres. There are three main types:

• Collagenous fibres: Made of collagen and consist of bundles of fibrils that are coils of collagen molecules.
• Elastic fibres: Made of elastin and are "stretchable."
• Reticular fibres: Consist of one or more types of very thin collagen fibres. They join connective tissues to other tissues.

Areolar Tissue

Areolar tissue is a common type of loose connective tissue and the most widely distributed type in vertebrates. Its fibres are far enough apart to leave ample open space for interstitial fluid. This tissue is strong enough to bind different tissue types together, yet soft enough to provide flexibility and cushioning. It exhibits interlacing, loosely organised fibres, abundant blood vessels, and significant empty space filled with interstitial fluid.

Function

Areolar connective tissue holds organs in place and attaches epithelial tissue to other underlying tissues. It also serves as a reservoir of water and salts for surrounding tissues. Almost all cells obtain their nutrients from and release their wastes into areolar connective tissue.

Location

Loose connective tissue may be found in tissue sections from almost every part of the body. It surrounds blood vessels and nerves and penetrates with them even into the small spaces of muscles, tendons, and other tissues. Nearly every epithelium rests on a layer of areolar tissue, whose blood vessels provide the epithelium with nutrition, waste removal, and a ready supply of infection-fighting leukocytes when needed. The lamina propria of mucous membranes, such as those of the respiratory and alimentary systems, contains large numbers of immune cells. The areolar tissue is also found in the mesentery, surrounding the intestine.

Clinical Significance

Organs rich in loose connective tissue, such as the eyelids, are prone to oedema, a condition often indicating kidney failure or nephrotic syndrome. Periorbital swelling is a characteristic finding in severe kidney disease.

Loose connective tissue is the initial site where pathogenic agents that breach an epithelial surface are challenged and destroyed by immune system cells. This tissue's primary function is as a site of inflammatory and immune reactions. Large populations of immune cells are maintained in areas of the body where foreign substances are continually present.

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Self-assessment MCQs (single best answer)

What is another name for loose connective tissue?

What type of cells secrete collagen proteins in loose connective tissue?

Which of the following is NOT a function of loose connective tissue?

What type of fibres in loose connective tissue are "stretchable"?

Where is areolar connective tissue commonly found?

Which cell type in loose connective tissue plays a role in inflammatory responses by releasing histamine?

What is the primary role of macrophages in loose connective tissue?

What component of loose connective tissue occupies more volume than the fibres?

In what condition are organs rich in loose connective tissue, such as the eyelids, prone to oedema?

Which type of fibre in loose connective tissue consists of bundles of fibrils that are coils of collagen molecules?